CdO-ThO2 -Free, highly refractive optical glass

ABSTRACT

CdO- and ThO-free, highly refractive optical glass having a refractive index of 1.85-2.05 and an Abbe index of 25-43, characterized by the following composition (in percent by weight): -SiO2 5-15 -B2O3 2.5-8 -SiO2 + B2O3 7-23 -SiO2/B2O3 1.3-2.1 -La2O3 43-56 -Gd2O3 0-14 -La2O3 + Gd2O3 43-60 -ZrO2 3-10 -Nb2O5 0-15 -Ta2O5 0-20 -Nb2O5 + Ta2O5, 11-22 -

TECHNICAL FIELD OF THE INVENTION

The present invention relates to highly and maximally refractive opticalglasses with a high Abbe index and to the manufacture of optical glasseswith previously unattained refractive indexes of up to 2.05 and Abbeindexes of >25.

BACKGROUND OF THE INVENTION

Japanese published patent application No. 79/133,506 describes glasseshaving refractive indeces of 1.90-1.95 and similar Abbe values asdescribed in the present invention; however, refractive values of onlyup to 1.95 can be achieved and these values only if a considerableamount of GeO₂ is employed, which is very expensive.

Glasses having refractive values of up to 1.95 and similar Abbe indexesare also described in Japanese published patent application 79/90,218;however, these familiar glasses contain either expensive GeO₂ or are notsufficiently stable towards devitrification.

DISCLOSURE OF THE INVENTION

The present invention relates to highly and maximally refractive opticalglasses with a high Abbe index and to the manufacture of optical glasseswith refractive indexes of up to 2.05 and Abbe indexes of greater than25.

One very important characteristic of the present invention is thesurprisingly high ratio of SiO₂ /B₂ O₃, which lies between 1.3 and 2.1in accordance with the invention. Most previously known glasses in thiselevated position in the nd-, vd-diagram have required formulas in whichthe B₂ O₃ component was greater than the SiO₂ component. For example,the maximum SiO₂ /B₂ O₃ ratio in glasses according to DOS 26/53,581 isequal to 1. In glasses according to DAS No. 1,926,959 with nd=1.9 andvd=40, the SiO₂ /B₂ O₃ ratio is approximately 0.75; moreover, theseglasses contain expensive GeO₂. The same holds true for glassesaccording to DOS 20/57,706.

Some other highly refractive optical glasses having an SiO₂ /B₂ O₃-ratio of ≧1 are described in DAS 1,061,976. All examples named thereinwith refractive indexes of >1.85 have La₂ O₃ components of about 63percent by weight. Such high concentrations of La₂ O₃ componentsdestabilize the glasses; additionally, these glasses must contain Al₂ O₃or Y₂ O₃.

The optimum SiO₂ /B₂ O₃ ratio according to the invention lies between1.6-1.9. To achieve the required high refractive indexes of ≧1.86, 43-56percent by weight of La₂ O₃ are required and, optionally, 0-14 percentby weight of Gd₂ O₃, with the sum of these two components amounting to43-60 percent by weight.

The amount of ZrO₂ is between 3-10 percent by weight. It is mandatory tohave 11-22 percent by weight of the pentavalent substitute glassproducers Ta₂ O₅ and Nb₂ O₅, whereby the component of Nb₂ O₅ may bebetween 0-15 percent by weight and that of Ta₂ O₅ between 0-20 percentby weight. Only with these high amounts of Nb₂ O₅ and/or Ta₂ O₅ canglasses of the required optical position be molten which aresufficiently stable to devitrification; additionally, these componentsincrease the refractive index considerably. Additionally, PbO, (0-13percent by weight), TiO₂ (0-13 percent by weight), WO₃ (0-23 percent byweight) and ZnO (0-3 percent by weight) can be added.

BEST MODE FOR CARRYING OUT THE INVENTION

Glasses in accordance with the present invention are produced in thefollowing manner: The starting materials (the appropriate oxides,nitrates, and carbonates) are weighed in accordance with the formula; asmall portion of refining agent, such as As₂ O₃, is added in smallamounts and everything is mixed well. This mixture is melted in aPt-crucible at 1450-1550 degrees C., is clarified and homogenized wellby means of an agitator. The mixture is then stirred until the castingtemperature of about 1400 degrees C. is reached, and is then cast in aniron form.

The table contains 11 compositions and their optical values by way ofexample:

                                      TABLE                                       __________________________________________________________________________    EXAMPLE 1    2    3    4    5    6    7    8    9    10   11                  __________________________________________________________________________    SiO.sub.2                                                                             10.80                                                                              5.40 7.10 10.90                                                                              7.35 10.35                                                                              7.75 7.45 12.70                                                                              14.55                                                                              7.40                B.sub.2 O.sub.3                                                                       6.35 3.00 4.40 6.55 4.35 5.80 5.85 4.25 6.90 7.10 4.20                La.sub.2 O.sub.3                                                                      54.85                                                                              49.45                                                                              47.90                                                                              47.75                                                                              43.80                                                                              50.10                                                                              55.30                                                                              54.25                                                                              45.65                                                                              46.30                                                                              53.50               ZrO.sub.2                                                                             8.50 5.75 5.35 5.80 6.20 9.60 8.40 3.65 7.60 7.05 7.50                Nb.sub.2 O.sub.5                                                                      4.35 8.55 8.35 7.10 5.40 6.50 9.70 9.60      11.60                                                                              9.47                Ta.sub.2 O.sub.5                                                                      13.50                                                                              9.45 12.55                                                                              8.35 7.65 10.50                                                                              11.00                                                                              10.90                                                                              16.35     10.75               PbO     1.00 11.95                                                                              1.75 2.45 2.25 4.70 1.30 3.60      0.75 3.55                TiO.sub.2                                                                             0.65 6.45 12.60                                                                              0.80 0.75      0.70 6.30      0.40 3.65                                       WO.sub.3                                                                           WO.sub.3                                                                           Zno            Gd.sub.2 O.sub.3                                                                   Gd.sub.2 O.sub.3                                10.30                                                                              22.25                                                                              2.45           10.80                                                                              12.25                    nd      1.9111                                                                             2.0488                                                                             2.0373                                                                             1.9189                                                                             1.9712                                                                             1.9255                                                                             1.9594                                                                             2.0430                                                                             1.8630                                                                             1.8780                                                                             1.9940              vd      38.50                                                                              27.10                                                                              26.75                                                                              34.50                                                                              30.75                                                                              35.60                                                                              35.00                                                                              30.10                                                                              42.10                                                                              38.20                                                                              31.40               SiO.sub.2/                                                                            1.70 1.8  1.61 1.66 1.69 1.78 1.32 1.75 1.84 2.05 1.76                B.sub.2 O.sub.3                                                               __________________________________________________________________________

What is claimed is:
 1. CdO- and ThO-free, highly refractive opticalglass which are stable to devitrification and have a refractive index ndof 1.85-2.05 and an Abbe index vd of 25-43, consisting essentially ofthe following composition (in percent by weight):

    ______________________________________                                        SiO.sub.2              5-15                                                   B.sub.2 O.sub.3        2.5-8                                                  SiO.sub.2 + B.sub.2 O.sub.3                                                                          7-23                                                   SiO.sub.2 /B.sub.2 O.sub.3                                                                           1.6-1.9                                                La.sub.2 O.sub.3       43-56                                                  Gd.sub.2 O.sub.3       0-14                                                   La.sub.2 O.sub.3 + Gd.sub.2 O.sub.3                                                                  43-60                                                  ZrO.sub.2              5-9                                                    Nb.sub.2 O.sub.5       0-15                                                   Ta.sub.2 O.sub.5       0-20                                                   Nb.sub.2 O.sub.5 + Ta.sub.2 O.sub.5                                                                  11-22                                                  ______________________________________                                    


2. Glass as claimed in claim 1, further containing 0.4-13 percent byweight of TiO₂.
 3. Glass as claimed in claim 1, further containing0.75-13 percent by weight of PbO.
 4. Glass as claimed in claim 1,further comprising 10-23 percent by weight of WO₃.
 5. Glass as claimedin claim 1, further containing 1-3 percent by weight of ZnO.